1. Field of the Invention
This invention has relation to the mooring of floating vessels over predetermined locations on the floors of bodies of water. More specifically, it relates to the holding and rapid release of wire rope mooring lines, as well as the rapid paying out and drawing in of extensive lengths of wire rope mooring lines under varying loads.
2. Description of the Prior Art
It is known to anchor semi-submersible offshore drilling platforms in place utilizing elongated anchor lines extending out from such platforms in several directions. See U.S. Pat Nos. 3,842,776, 3,318,275 and 3,349,740. Semi-submersible oil exploration and drilling platforms include large horizontal floats or pontoons that are kept entirely air-filled while they are being towed into place to cause the platforms to float on the surface of the water. In order to minimize the effects of ocean swells and waves on the platforms during use of the platforms for their intended purposes, the pontoons are filled with water and submerged. This subjects only the narrow main vertical support columns to the surface action of the sea.
Relatively high tensile forces in the mooring lines are necessary for stable mooring. Once a platform is in its desired position, the sea water can be let into the pontoons to achieve the desired submersion of the platform. Winches can be used to draw in the lines at whatever force the winches can deliver. Then, in the process of fixedly mooring the platform, the water can be evacuated from the pontoons, leaving the mooring lines passing through fairleads located at lower portions of the platform to restrain the platform from rising as the pontoons become more and more buoyant. This has the effect of positively and firmly fixing the positioning of the platform over a drilling point, for example, and substantially diminishes the effects of surface action of the seas on the platform. After and as the pontoons are evacuated, extreme upward forces are being absorbed by the mooring lines.
In floating offshore drilling platforms and in similar vessels, deck space and other working space are at a premium. The economic effectiveness of a particular vessel is measured in proportion to the amount of space available to perform the primary function of the vessel (drilling for oil, for example) and in reverse proportion to the amount of space required for the necessary subsidiary functions (holding the vessel in place and handling and storing the mooring lines, for example). Also important is the weight of deck-located mooring equipment, because the vessel may have limited structural or carrying capacity, most of which must be dedicated to its primary functions.
In the past, mooring lines have been handled primarily on large winch-driven single drums. Such a drum must often carry extensive lengths of the line it holds. It must also be able to pull with sufficient force to draw up a mooring line to the desired tension, often several hundred thousand pounds of force. The same line-carrying drum, equipped with a braking system, is usually also required to provide the braking or holding force after the desired mooring tension is achieved.
Good design and recent safety regulations in some jurisdictions have added a new requirement for mooring systems, namely, the ability to quickly release and pay out the moored line in certain emergency situations. For example, when there is a release of gas below the surface, the buoyancy of the platform is reduced by all the gas dissolved in the surrounding water. This reduced buoyancy can cause the platform to sink rapidly. One solution is the rapid release of one or more mooring lines on one side of the platform, so that the mooring forces in the remaining lines can quickly pull the platform away from its moored position.
When a single winch-driven drum is used, the larger the drum, the more difficult it is for the drum to provide the necessary pulling and holding forces under conditions where a great deal of line is carried on the drum. As the distance of the outer wrap of the line relative to the drum axis increases, the torque necessary to place a given tensile force in the line increases. As greater tensile forces are exerted, there is an increased tendency of the last wrap of the line to force itself in between the inner wraps of the line, causing irregular winding of the line. The preceding problems would become particularly acute if safety design requirements were to demand that a winch-driven drum hold or pull forces approaching the breaking strength of the mooring lines. Usually, the breaking strength is significantly greater than the typical forces pulled by the winch when a mooring line and anchor are set. These problems are also acute when a vessel made for deep-water mooring is moved to shallower water, where a large portion of each mooring line is not paid out and must be stored in wraps on each winch. This large load of line severely reduces the winch's ability to pull and hold high mooring forces, because the winch is always pulling at a relatively large distance from it drum axis.
While winch-driven drums and their brakes and power units can be made larger and more powerful to deliver the required pulling and holding forces, they become increasingly expensive and heavy and take up more and more valuable deck space and carrying capacity. Their size and inertia also make quick release mechanisms for the drums difficult to design. Also, in retrofit situations, increasing the size of winches and related units may be limited by pre-existing design constraints.
Retrofitting mooring systems to increase mooring capacity is not uncommon. Improvements in wire rope design have permitted wire rope of a given size to have up to 40 percent greater breaking strength. But more powerful (and larger) winches are usually necessary to take advantage of this added strength. Increased safety requirements or design limitations on vessel excursion in storm situations may also make it desirable to upgrade mooring systems on existing vessels. Again, retrofitting with larger and more powerful conventional winch equipment offers one solution but leads to the problems discussed. Additional lines can also upgrade mooring capability, but these, too, use up deck space and carrying capacity.
One alternative to winch-driven drums is a linear pulling machine, such as that sold under the trademark "Lucker" by American Hoist & Derrick Company, Marine/Energy Division, St. Paul, Minn. Such machines use two Lucker wedge grip assemblies, one of which can be reciprocated relative to the other to pull successive short lengths of line with high forces. But these machines, while capable of pulling high forces and holding to the breaking strength of a line, are not suited for rapid pulling. Also, because their holding force on a line depends, in part, on tesion in the line, they pose special problems for rapid release. In particular, a Lucker grip, when holding a line under tension, must have all or substantially all of that tension removed before the grip wedges can be released.
U.S. Pat. No. 4,446,807 shows a mooring system using both a winch-driven drum and a Lucker pulling machine. The former is used for all line handling and pulling, except pulling involving the highest line tension, where the latter is employed.
The grip wedges of the Lucker machine in U.S. Pat. No. 4,446,807 can not only be used in line pulling but can also provide line holding. This is advantageous, because when a Lucker grip holds the line, the load is taken off other components. With a Lucker grip located above the fairlead, the winch and its brake and bearings are relieved of load, saving wear and tear and facilitating inspection, adjustment and repair. In some installations, it would even be possible to move a winch to another position or use it for another purpose while the Lucker grip holds the line. But use of a Lucker grip to hold a mooring line poses a dilemma, because a Lucker grip is capable of holding forces greater than those drveloped by either the winch-driven drum or the pulling section of the linear pulling machine in a mooring system such as in U.S. Pat. No. 4,446,807. If a Lucker grip is on a line and the line tension increases, due to a storm or raising of the platform by change in buoyancy, so that line tension exceeds the pulling capability of the winch-driven drum and the pull section of the Lucker machine, then the line cannot be released until tension decreases (except by cutting, in which case the mooring is irretrievably lost). U.S. Pat. No. 4,446,807 provides no mechanism for releasing a Lucker grip when tension of the line in the grip cannot be relieved.
Even in situations where a winch-driven drum or the pulling section of a linear pulling machine can pull with enough force to relieve the tension on a line held by a Lucker grip, readying the equipment to deliver this force may take time. In an emergency, quick release of the line is highly desirable.
A solution that has been proposed is an over-center trip mechanism that permits a Lucker grip in a linear pulling machine to move a short distance in the direction of the mooring lines's pull to come into contact with a pair of projections that prevent motion of the grip wedges holding the line. Although the grip wedges are stopped, the body of the Lucker grip will continue to travel a brief distance beyond the stopping point of the grip wedges. Due to the grip wedges' construction, this relative motion between the grip wedges and the grip body causes the grip wedges to release the line. A serious drawback of this design is that it relies on momentum of the grip body to separate the grip body and the grip wedges. This momentum may not be reliable and will be less effective in horizontal positions of a Lucker grip, where gravity will not help accelerate the grip body. If friction or some other obstructive force intervenes, there is no positive action to separate the grip body and its grip wedges to causes line release.
What is needed in the prior art is mooring apparatus that permits positive and quick release of mooring lines, that can deliver required pulling forces and pull at high speeds and that permits holding to the breaking strength of a line.